The present invention relates to method and apparatus for transmitting optical signals in high-capacity, long distance, lightwave communication systems which use Return-To-Zero (RZ) modulation format and dispersion management.
Optical nonlinearities of optical transmission fibers have become limiting factors for long distance, high capacity lightwave transmission systems. In an optically amplified transmission system transmission system, the amplified spontaneous emission (ASE) noise degrades the signal-to-noise ratio (SNR), and a higher signal power is then required to maintain a minimum SNR. However, optical nonlinearities distort the transmission signal and thus limits the maximum optical power that can be launched over the optical transmission line.
It is possible to balance the self-phase-modulation (SPM) with chromatic dispersion by a proper design of the transmission pulse waveform as well as the pulse energy. Such nonlinear pulses are known as xe2x80x9coptical solitonsxe2x80x9d. Since the chromatic dispersion is compensated for by optical nonlinearity, there is no need to perform dispersion compensation in soliton systems. In this regard see, for example, U.S. Pat. No. 4,558,921 (A. Hasegawa), issued on Dec. 17, 1985, U.S. Pat. No. 4,700,339 (J. P. Gordon), issued on Oct. 13, 1987, and U.S. Pat. No. 5,035,481 (L. F. Mollenauer) issued on Jul. 30, 1991.
Although transoceanic soliton transmission is known, conventional soliton transmission technology has not been commercialized. One of the major problems with conventional soliton transmission is timing jitter. A soliton pulse width is typically approximately 10% of a bit period and there is no fundamental mechanism that can fix such short pulses in time. Perturbations such as solitonxe2x80x94soliton interaction, frequency shift due to the ASE noise, and an acoustic wave generated by a previous pulse tend to move the pulses out of their original position as is indicated in U.S. Pat. No. 5,710,649 (L. F. Mollenauer), issued on Jan. 20, 1998. Still further, many techniques have been used to reduce or eliminate timing jitter as is described in the book entitled xe2x80x9cOptical Fiber Telecommunications IIIAxe2x80x9d by I. P. Kaminow et al., at pages 373-461 (Academic Press, 1997). All the above techniques, also known as xe2x80x9csoliton control technologiesxe2x80x9d are typically not cost effective in many practical applications, and also complicate system designs.
Recently, a new class of solitons (dispersion managed solitons) were described in the article entitled xe2x80x9cReduction of Gordon-Haus timing jitter by periodic dispersion compensation in Soliton transmissionxe2x80x9d by M. Suzuki et al., Electronic Letters, Vol. 31, No. 23, pages 2027-2029, 1995. An article entitled xe2x80x9cSoliton Transmission Using Periodic Dispersion Compensationxe2x80x9d, by N. J. Smith et al. in Journal of Lightwave Technology, Vol. 15, No. 10, pages 1808-1822, 1997, discusses a dispersion managed soliton (DMS) which has been shown to have a much better performance than a conventional soliton, while at the same time having inherent desired properties of conventional solitons in dealing with optical nonlinearities. There are five major improvements in DMS transmission systems compared to conventional soliton systems. These five improvements are (a) energy enhancement wherein it is possible to launch much higher signal power for a DMS signal than conventional soliton signals, which improves the system SNR; (b) reduced timing jitter; (c) no additional soliton control technologies are required; (d) it is compatible to existing non-return-to-zero (NRZ); and (e) it has high power and dispersion tolerance.
In an article entitled xe2x80x9c1 Tbit/s (100xc3x9710.7 Gbits) Transoceanic Transmission Using 30 nm-Wide Broadband Optical Repeaters with Aeff-Enlarged Positive Dispersion Slope Fiber and Slope-Compensating DCFxe2x80x9d by T. Tsuritani et al., at pages 38-39 of Post-Deadline Papers, 25th European Conference on Optical Communications, 1999, discloses that significant system performance has been achieved using DMS technology in many laboratory experiments in terms of both distance and total capacity. Similar performance has also been demonstrated by many other such as, for example, the articles entitled xe2x80x9c1 Terabit/s WDM Transmission over 10,000 kmxe2x80x9d by T. Naito et al., PD2-1, pages 24-25, 25th European Conference on Optical Communication, 1999, and xe2x80x9c1.1-Tb/s (55xc3x9720-GB/s) DENSE WDM SOLITON TRANSMISSION OVER 3,020-km WIDELY-DISPERSON-MANAGED TRANSMISSION LINE EMPLOYING 1.55/1.58 um HYBRID REPEATERSxe2x80x9d, by K. Fukuchi et al., PD2-10, pages 42-43, 25th European Conference on Optical Communication, 1999.
Although previous laboratory experiments have proven the feasibility of DMS systems for long distance and high capacity applications, there are many challenges to achieve both reliability and flexibility such that such DMS system can be used in a realistic environment. For example, in terrestrial optical fiber networks, the distance between repeaters or optical amplifiers can be as long as 130 km. Such distances are significantly longer than those of the previous experiments. The impacts of the longer repeater spacing are two-fold. First, the required signal power is typically much higher to overcome the SNR degradation caused by a required larger amplifier gain. Second, as a result of the required higher signal power, optical nonlinearities become more important for long distance transmission. Another challenge is that there are a variety of different optical fiber types in existing optical fiber networks. Optical nonlinearities become even more detrimental for a transmission line comprising different types of optical fibers. As for DMS systems, the total capacity is limited not only by the optical amplifier bandwidth, but also the higher order chromatic dispersion of the transmission fiber. The situation is even more difficult when there are several different types of optical fibers involved. A reliable and cost-effective solution to higher order chromatic dispersion is one of the major challenges for high capacity long haul DMS systems. Finally, distance and capacity are not the only requirements for next generation optical networks. For example, it is highly desirable to have the flexibility to place optical add/drop nodes anywhere along an optical transmission line.
In a dispersion managed soliton (DMS) system, DMS predicts significant power enhancement which is valid for single channel propagation. The power enhancement cannot be fully utilized for multi-channel system since the cross-phase modulation (XPM) dominates the overall system performance. DMS systems further require an accurate balance between the self-phase modulation (SPM) in the transmission fiber and the SPM a dispersion-compensating fiber which often results in a much smaller system margin.
It is desirable to provide a high capacity ultra-long haul dispersion and nonlinearity managed lightwave communication system which overcomes the problems described hereinabove.
The present invention is directed to method and apparatus for transmitting optical signals in high-capacity, long distance, lightwave communication systems which use Return-To-Zero (RZ) modulation format and dispersion management.
Viewed from an apparatus aspect, the present invention is directed to an optical transmission system. The optical transmission system comprises a transmitter terminal and an optical transmission line. The transmitter terminal comprises a plurality of return-to-zero (RZ) transmitters and a multiplexing arrangement. Each of the plurality of return-to-zero (RZ) transmitters is adapted to receive a separate channel input signal and to generate therefrom a corresponding forward error correction (FEC) modulated output signal in a separate predetermined channel frequency sub-band of an overall frequency band which includes a predetermined channel separation from an adjacent channel frequency band generated by another RZ transmitter. The multiplexing arrangement multiplexes the plurality of the predetermined channel frequency sub-bands from the plurality of RZ transmitters into separate groups of frequency bands where the groups of frequency bands have a predetermined band-gap separation therebetween wherein each group of frequency bands has a predetermined separate pre-chirp introduced before being multiplexed with all other groups of frequency bands into a single multiplexed output signal. The optical transmission line has an input coupled to an output of the multiplexing arrangement and is subdivided into a plurality of optical transmission line sections. The optical transmission line comprises a plurality of Raman amplifiers and at least one dispersion compensating line amplifier (DCLA). One of the plurality of Raman amplifiers is located at the end of each optical transmission line section and is adapted to receive at an input thereof the single multiplexed output signal propagating in an associated transmission line section and to combine a predetermined Raman pump power signal into the optical transmission line section in an opposite direction to the received single multiplexed output signal to generate at an output thereof an output signal which is Raman amplified for increasing a path averaged optical power without increasing nonlinear degradation. The at least one DCLA is coupled to an output of a Raman amplifier at the end of a predetermined group of optical transmission line sections. The DCLA is adapted to introduce dispersion compensation for the single multiplexed output signal, and to introduce separate high-order dispersion compensation for each of the groups of frequency bands therein.
Viewed from a method aspect, the present invention is directed to a method of transmitting signals in an optical transmission system. The method comprises the steps of: (a) receiving each channel input signal of a plurality of channel input signals in a separate one of a plurality of return-to-zero (RZ) transmitters, and generating therefrom a corresponding forward error correction (FEC) modulated output signal in a separate predetermined channel frequency sub-band of an overall frequency band which includes a predetermined channel separation from an adjacent channel frequency band generated by another RZ transmitter; (b) multiplexing the plurality of the predetermined channel frequency bands from the plurality of RZ transmitters into separate groups of frequency bands where the groups of frequency bands have a predetermined band-gap separation therebetween, wherein each group of frequency bands has a predetermined separate pre-chirp introduced before being multiplexed with all other groups of frequency bands into a single multiplexed output signal; (c) receiving the single multiplexed output signal in an optical transmission line which is subdivided into predetermined optical transmission line sections; (d) receiving the single multiplexed output signal propagating in each optical transmission line section by a separate Raman amplifier which combines a predetermined Raman pump power signal into the optical transmission line section in an opposite direction to the received single multiplexed output signal to generate an output signal which is Raman amplified for increasing a path averaged optical power without increasing nonlinear degradation; and (e) introducing dispersion compensation from a dispersion compensating line amplifier (DCLA) into the single multiplexed output signal from an output of a Raman amplifier at the end of a predetermined group of optical transmission line sections for providing separate high-order dispersion compensation for each of the groups of frequency bands in said single multiplexed output signal.